This invention relates to ink jet print heads and, more preferably, to print heads having improved durability.
Ink jet print heads are sandwich structures having top and bottom plates, and a dry film photoresist intermediate layer that defines liquid pathways and discharge orifices. The top plate contains the ink nozzles, and typically is made of a noble metal, glass or plastic. The bottom plate typically is a thermally stable substrate, such as a silicon wafer, that bears microcircuits. Microresistors are mounted on the substrate, projecting into the liquid pathways in the photoresist layer, in alignment with the ink nozzles. At computer command, the resistors superheat nearby ink, creating a steam bubble that forces ink droplets out the nozzles.
The dry film photoresist layer must meet many demanding requirements. It must be capable of being imaged to the fine resolution needed to define the ink passageways. The layer must be dimensionally stable (e.g., not swell) and capable of withstanding chemical attack from the hot aqueous inks, which typically have high pH and contain organic components. Moreover, the layer must remain firmly bonded to the top and bottom plates, which frequently are constructed of materials difficult to bond to (e.g., gold), during millions of firing cycles, despite stresses that tend to cause delamination.
Various photoresist materials have been proposed to meet the demanding requirements of print head construction. For example, Japanese Patent Application 5-278222, published Oct. 26, 1993, discloses a dry photoresist film containing a half acryloyl ester of a bis-phenol A epoxy monomer, a photoinitiator, and a polymeric binder that is said to be particularly useful for this purpose. Vacrel(copyright) solder mask material, a photoresist material sold by E. I. du Pont de Nemours and Company that contains acrylic and melamine monomers, photoinitiator, and an acrylic binder with acidic functions for water-based development, also has been used for this purpose.
The inks contained in the print heads contain cosolvents to improve their daytime and water fastness. However, these cosolvents have a tendency to swell known resist layers causing them to delaminate from the top and bottom plates, leading to failure of the print head.
Although known dry film photoresists have proven useful, further improvements are desired to achieve the desired durability for extended use of ink jet print heads.
It now has been found that the photoresists containing poly(amic acid) will improve durability of the print heads. Accordingly, the present invention provides an ink jet print head having a top plate, an intermediate photoresist layer defining ink passageways, and a bottom plate, wherein the photoresist layer, prior to imaging to form the passageways, contains an ethylenically unsaturated compound, a photoinitiator, and a poly(amic acid) prepared from at least one dianhydride and at least one aliphatic diamine.
The photopolymerizable poly(amic acid) solutions can also be used to make solder masks, photoimageable coverlays, for flexible circuits, optical wave guides and an adhesiveless laminate to make disk drives.
While the details of print head design will vary with the manufacturer, the print head 10 (as seen in FIG. 1) generally has a top plate, 11 (which is shown partly sectioned to permit illustration of the ink passageways) an intermediate photoresist layer 12, and a bottom plate 13. The intermediate photoresist layer 12 is a dry film that is imaged during the manufacturing process, followed by removal of non-exposed regions of the photoresist, to form ink passageways 14, 15. It is critical that the photosensitive or photoresist layer 12 remain firmly bonded to the top 11 and bottom 13 layers of the print head 10 during extended use, and be resistant to chemical attack or swelling by the inks, which are aqueous based and typically contain organic components.
The bottom plate serves as a mounting platform for microresistors or other elements used to generate pressure to discharge the ink, such as heat generating or piezo elements. The bottom plate typically is constructed of silicon, glass, ceramic, plastic or metal. Sputter-coated xe2x80x9cpassivationxe2x80x9d layers may be employed to protect circuits mounted on the bottom plate from ink attack. For example, passivation layers of Si3N4 and SiC are shown in U.S. Pat. No. 4,809,428. Other inorganic oxide or inorganic nitride materials useful for this purpose are SiO2, Ta2O5, Al2O3, glass, BN, etc.
The bottom plate also may be coated with a metal protective layer to impart ink resistance, either with or without a passivation layer. Anti-corrosive metals such as Ti, Cr, Ni, Ta, Mo, W, Nb and the like may be selected for this purpose, or alloys such as stainless steel or noble metals. Noble metals such as gold, palladium or platinum also may be selected, but are more difficult to bond to the photoresist layer.
The photoresist layer may be applied to the bottom plate either in liquid form, then dried, or preferably as a dry film. The photoresist layer contains an ethylenically unsaturated compound or monomer, a photoinitiator, and a poly(amic acid) prepared from at least one dianhydride and at least one aliphatic diamine in accordance with this invention. Other additives may be present to modify the properties of photopolymer materials. Particularly preferred composites are disclosed in U.S. Pat. No. 4,937,172, and are composed of a monomer that is a half acryloyl ester of bisphenol A epoxy; a photoinitiator system; and a macromolecular elastomeric water-insoluble binder.
Poly(amic acid):
Poly(amic acid) can be easily prepared by reacting a tetracarboxylic dianhydride to at least one aliphatic diamine in an organic solvent.
The tetracarboxylic dianhydride may be represented by the formula: 
wherein R1 is a tetravalent organic group having 2 to 30 carbon atoms, preferably 6-21 carbon atoms. For example, aliphatic or alicyclic tetracarboxylic dianhydrides can be used to practice the invention. Useful examples include but are not limited to butanetetracarboxylic dianhydride, pentane-tetracarboxylic dianhydride, hexanetetracarboxylic dianhydride, cyclobutanetetracarboxylic dianhydride, cyclopentanetetracarboxylic dianhydride, bicyclohexanetetracarboxylic dianhydride, cyclopropanetetracarboxylic dianhydride, and methylcyclohexanetetracarboxylic dianhydride, ethylene diamine tetraacetic dianhydride, propylene diamine tetraacetic dianhydride and bicyclo octene tetracarboxylic anhydride. The use of an aromatic tetracarboxylic dianhydride can afford a poly(amic acid) which can be converted to a polyimide with superior physical properties. Examples of useful aromatic tetracarboxylic dianhydrides include but are not limited to 2,2xe2x80x2-bis(3,3xe2x80x2,4,4xe2x80x2-biphenyl)-hexaflurorpropane dianhydride (6-FDA), 3,3xe2x80x2,4,4xe2x80x2-benzophenonetetracarboxylic dianhydride (BTDA), pyromellitic dianhydride (PMDA), oxydiphthalic anhydride (ODPA), 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxlic dianhydride, 2,2xe2x80x2,3,3xe2x80x2-biphenyltetracarboxlic dianhydride, 2,3,3xe2x80x2,-4xe2x80x2-biphenyltetracarboxylic dianhydride, 2,2-bis[5-(3,3-dicarboxyphenoxy)phenyl]propane dianhydride, 2,2-bis(3,-4-dicarboxy-phenyl)propane dianhydride, 2,2xe2x80x2-bis(3,3xe2x80x2,4,-4xe2x80x2-biphenylpropane dianhydride, bis(3,4xe2x80x2-dicarboxyphenyl) dimethylsilane dianhydride, bis(3,4-dicarboxyphenyl) dimethylsiloxane dianhydride, 4,4xe2x80x2-isophthaloyl diphthalic anhydride, hydroquinone diether anhydride, sulfonyl bis(phthalic anhydride), and ethyleneglycol bis(trimellitic anhydride), 3,4,9,10-perylenetetracarboxylic dianhydride, 4,4xe2x80x2-sulfonyldiphthalic dianhydride, 3,3xe2x80x24,4xe2x80x2biphenyltetracarboxylic dianhydride (BPDA), 1,2,5,6-naphthalenetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, and 2,3,5,6-pyridinetetracarboxylic dianhydride, and mixtures thereof. Many of the dianhydrides are commercially available. The diamine used in the present invention may be represented by the formula:
H2Nxe2x80x94R2xe2x80x94NH2
wherein R2 is a divalent organic group having 1 to 30 carbon atoms, preferably 4 to 18 carbon atoms and more preferably 6 to 12 carbon atoms. Useful examples include but are not limited to 4,4xe2x80x2-oxydianiline (ODA), 4,4xe2x80x2-diaminodiphenylmethane, 4,4xe2x80x2-diaminodiphenylsulfone, 4,41-diaminodiphenyl sulfide, m-phenylenediamine (MPD), p-phenylenediamine, 2,4-diaminotoluene, 2,5-diaminotoluene, 2,6-diaminotoluene, benzidine, 3,3xe2x80x2-dimethylbenzidine, 3,3xe2x80x2-dimethylbenzidine, 3,3xe2x80x2-dimethoxybenzidine, o-tolidine, 4,4xe2x80x3-diaminoterphenyl, 1,5-diaminonaphthalene, 2,5-diaminopyridine, 3,3xe2x80x2-dimethyl-4,4xe2x80x2-diaminodiphenylmethane, 4,4xe2x80x2-bis(p-aminophenoxy)biphenyl, 2,2-bis[4-(p-aminophenoxy)phenyl]propane, and hexahydro-4,7-methanoindanylene dimethylenediamine, 4,4xe2x80x2-diaminodiphenylhexafluoropropane, bisaniline p, bisaniline m, 1,5-naphthalene diamine, 1,4-bis(aminophenoxy) benzene, 2,2-bis[4-(4-aminophenoxyphenyl]propane, 4,4xe2x80x2-(bisaminophenoxy) biphenyl,ethylene dianiline, ethylene diamine, 1,3-diaminopropane, 1,3-diamino-2-hydroxypropane, 1,4-diaminobutane, hexamethylene diamine, 2-methyl-1,5-pentane diamine, 1,7-diaminoheptane, 1,9-diaminononane, 1,10-diaminodecane, 1,12,-dodecane diamine, dodecafluoro dodecane diamine, Jeffamine(copyright) D-230, Jeffamine(copyright) D-400, Jeffamine(copyright) D-2000, Jeffamine(copyright) EDR-148, Jeffamine(copyright) ED-600, and mixtures thereof. The Jeffamines(copyright) are commercially available from The Texaco Chemical Company, Athens, N.Y.
Some suitable aliphatic diamines and dianhydrides are disclosed in U.S. Pat. No. 5,348,835.
The use of a siloxanediamine represented by the following formula can improve the adhesion to an organic substrate: 
wherein R3 is a divalent organic group having 1 to 10 carbon atoms, R4, R5, R6 and R7 which may be the same or different, are each a monovalent organic group having 1 to 10 carbon atoms, and m is an integer of 1 to 10. The siloxane diamine is used usually in an amount of 1 to 20 mole percent of the total amount of diamine. If the concentration of siloxanediamine is too low, it may not produce the desired improvement on adhesion. If the concentration is too high, the heat resistance will degrade. The proper amount depends on the system and the substrate used, and it can be experimentally obtained. An example of a useful siloxanediamine is bis-3-(aminopropyl)tetramethylsiloxane.
The organic solvent used for the synthesis is preferably also a good solvent for the resulting poly(amic acid). Commonly used are the polar solvents such as, for example, N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylforamide, dimethyl sulfoxide, tetramethylurea, N-methylcaprolactam, Gama-caprolactone, sulfolane, toluene, xylene, 2-pyrrolidone or hexamethylphosphoric triamide.
The poly(amic acid) may be isolated for later use or preferably used as is prepared in solution form. Some poly(amic acids) described above are commercially available from E. I. Du Pont (Wilmington, D.E.) under the trade name of Pyralin(copyright).
The number of acid groups on the poly(amic acid) may be reduced to improve physical properties for handleability and moisture sensitivity during storage, or to obtain better development latitude. This can be accomplished by partial imidization, partial esterification, or use of blends of poly(amic acid) with its ester derivatives. The poly(amic acid) polymer may be present in the amount of 30 to 90%, preferably 40 to 70%, based on the total weight of the photosensitive or photoresist layer. Photoresist layers containing the poly(amic acids) exhibit durable bonds to the top and bottom plates of the print head, improved shelf life, and excellent ink resistance
Monomers:
Conventional monomers used in photosensitive resist compositions may be selected in practicing the invention. The selected monomer will preferably contain at least one ethylenically unsaturated groups capable of undergoing polymerization upon exposure to actinic radiation. Excessive amounts of trifunctional acrylate monomers should be avoided as it may cause undue reduction in flexibility.
Some suitable monomers which can be used as the sole monomer, or used in combination with others, include acrylate and methacrylate derivatives of alcohols, isocyanates, esters, epoxides and the like. Examples are diethylene glycol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyoxyethylated and polyoxypropylated trimethylolpropane triacrylate and trimethacrylate and similar compounds as disclosed in U.S. Pat. No. 3,380,831,2-hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, N-methyl diethanolamine dimethacrylate, hexamethylene glycol diacrylate, triethylene glycol diacrylate, tripropylene glycol diacrylate, trimethylolpropane triacrylate, polyoxyethylated trimethylolpropane triacrylate, polyoxypropylated trimethololpropane triacrylate, pentaerythritol tri-and tetracrylate, aromatic urethane diacrylates available from Sartomer, West Chester, Pa., bisphenol-A diacrylate, di-(3-acryloxy-2-hydroxypropyl) ether of bisphenol-A, di-(3-acryloxy-2-hydroxypropyl) ether of tetrabromo-bisphenol-A, or methacrylate analogues thereof as well as aliphatic urethane diacrylates such as those sold by Sartomer, di-(3-methacryloxy-2-hydroxypropyl) ether of tetrachlorobisphenol-A, di-(2-methacryl-oxyethyl) ether of tetrachlorobisphenol-A, di-(3-methacryloxy-2-hydroxypropyl) ether of tetrabromo-bisphenol-A, di-(2-methacryloxyethyl) ether of tetrabromo-bisphenol-A, triethylene glycol dimethacrylate, trimethylol propane triacrylate, polycaprolactone diacrylate, and aromatic urethane oligomeric di(meth)acrylates such as those sold by Sartomer, West Chester, Pa.
A particularly preferred class of monomers is aminoacrylates such as dimethylamino ethylmethacrylate, 3-(N,N-dimethylamino)propyl methacrylate, dimethylaminoethyl acrylate, dimethylamino)propyl methacrylamide and 2-tert. butylaminoethyl methacrylate.
The monomer(s) typically will constitute 5 to 50%, preferably 20 to 40%, by weight of the total weight of the photoresist layer.
Photoinitiator:
Conventional photoinitiators, or photoinitiator systems, may be selected in practicing the invention. The initiator directly furnishes free radicals when activated by actinic radiation. A sensitizer also may be present, typically to extend spectral response into the near ultraviolet, visible, and near infrared spectral regions.
Sensitizers which improve photospeed when used in combination with the phosphorous containing compound include the bis(p-dialkylaminobenzylidene) ketones disclosed in Baum et al., U.S. Pat. No. 3,652,275, and the arylidene aryl ketones disclosed in Dueber, U.S. Pat. No. 4,162,162. Some other initiators which improve photospeed when used in combination with the phosphorous containing compound include hydrogen donor compounds that function as chain transfer agents in the photopolymer compositions include: 2-mercapto-benzoxazole, 2-mercaptobenzothiazole, 4-methyl-4H-1,2,4-triazole-3-thiol, etc.; as well as various compounds disclosed in column 12, lines 18 to 58 of MacLachlan, U.S. Pat. No. 3,390,996. Suitable hydrogen donor compounds for use in systems containing both biimidazole type initiator and N-vinyl carbazole are 5-chloro-2-mercaptobenzo-thiazole; 2-mercaptobenzo-thiazole; 1H-1,2,4-triazole-3-thiol; 6-ethoxy-2-mercaptobenzothiazole; 4-methyl-4H-1,2,4-triazole-3-thiol; 1-dodecane-thiol; and mixtures thereof.
Preferred photoinitiator systems, which improve photospeed alone or in combination with the phosphorous containing compound, include 2,4,5-triphenylimidazolyl dimers in combination with chain transfer agents, or hydrogen donors, such as those disclosed in U.S. Pat. Nos. 3,479,185; 3,784,557; 4,311,783; and 4,622,286. Preferred hexaarylbiimidazoles (HABI) are 2-ortho-chlorosubstituted hexaphenylbiimidazoles in which the other positions on the phenyl radicals are unsubstituted or substituted with chloro, methyl or methoxy. The most preferred initiator is ortho-Cl-HABI, i.e., 1,1xe2x80x2-biimidazole, 2,2xe2x80x2-bis(ortho-chlorophenyl)-4,4xe2x80x2,5,5xe2x80x2-tetraphenyl-imidazole dimer.
A large number of free-radical generating compounds, including redox systems such as Rose Bengal/2-dibutylaminothanol, may be selected to advantage. Sensitizers useful with photoinitiators include methylene blue and those disclosed in U.S. Pats. Nos. 3,554,753; 3,563,750; 3,563,751; 3,647,467; 3,652,275; 4,162,162; 4,268,667; 4,351,893; 4,454,218; 4,535,052; and 4,565,769.
A particularly preferred class of photoinitiators and photosensitizers, which improve photospeed when used in combination with the phosphorous containing compound, are benzophenone, Michler""s ketone, ethyl Michler""s ketone, p-dialkylaminobenzaldehydes, p-dialkylaminobenzoate alkyl esters, polynuclear quinones, thioxanthones, hexaarylbiimidazoles, cyclohexadienones, benzoin, benzoin dialkyl ethers, or combinations thereof where alkyl contains 1 to 4 carbon atoms.
The photoinitiator, or photoinitiator system, typically will constitute 0.5 to 10%, preferably 2 to 5%, by weight of the total weight of the photoresist layer.
Additives:
Conventional additives incorporated in photosensitive compositions may be included. For example, the photoresist may contain binders, thermal polymerization inhibitors such as p-methoxy phenol, hydroquinone and alkyl and aryl-substituted hydroquinones and quinones, tertiary butyul catechol, pyrogallol, copper resinate, naphthylamines, betanaphthol, cuprous chloride, 2,6-di-ter-butyl-p-cresol, phenothiazine, pyridine, nitrobenzene, dinitrobenzene, p-toluquinone and chloranil. Also useful as thermal polymerization inhibitors are the nitroso compositions disclosed in U.S. Pat. No. 4,168,982. Various dyes and pigments may be added to increase the visibility of the resist image, provided the dye or colorant is transparent to the actinic radiation used to create the ink channels in the photoresist layer.
Suitable binders which can be used as the sole binder, or in combination with others, include the following: polyacrylate and alpha-alkyl polyacrylate esters (e.g., polymethyl methacrylate, polyethyl methacrylate, polybutyl methacrylate and polyhexyl methacrylate); copolymers and terpolymers of isobornyl acrylate, hydroxyethyl methacrylate, and butane diacrylate; copolymers of glycidyl ethers with the above mentioned monomers; polyvinyl esters (e.g., polyvinyl acetate, polyvinyl acetate/acrylate, polyvinyl acetate/methacrylate and hydrolyzed polyvinyl acetate); ethylene/vinyl acetate copolymers; polystyrene polymers and copolymers; saturated and unsaturated polyurethanes; synthetic rubbers (e.g., butadiene/acrylonitrile, acrylonitrile/butadiene/styrene and/or bromostyrene, methyl methacrylate/acrylonitrile/-butadiene styrene copolymers, 2-chlorobutadiene/1,3-polymers, and styrene/-butadiene/styrene, styrene/isoprene/styrene block copolymers); polybromostyrene; polyethylene oxides of polyglycols having average molecular weights from about 4,000 to 1,000,000; epoxides (e.g., epoxides containing acrylate or methacrylate groups); copolyesters, e.g., those prepared from the reaction product of a polymethylene glycol of the formula HO(CH2)nOH, where n is a whole number 2 to 10 inclusive, and (1) hexahydroterephthalic, sebacic and terephthalic acids, (2) terephthalic, isophthalic and sebacic acids, (3) terephthalic and sebacic acids, (4) terephthalic and isophthalic acids, and (5) mixtures of copolyesters prepared from said glycols and (i) terephthalic, isophthalic and sebacic acids and (ii) terephthalic, isophthalic, sebacic and adipic acids; nylons or polyamides, e.g., N-methoxymethyl polyhexamethylene adipamide; cellulose esters, e.g., cellulose acetate, cellulose acetate succinate and cellulose acetate butyrate; cellulose ethers, e.g., methyl cellulose, ethyl cellulose and benzyl cellulose; polycarbonates; polyvinyl acetal, e.g., polyvinyl butyral, polyvinyl formal; polyformaldehyde. The composition may also contain a preformed macromolecular elastomeric polymer binder similar to that disclosed in U.S. Pat. No. 4,937,172.
The binder(s) typically will constitute 0 to 50%, preferably 0 to 30%, by weight of the total weight of the photoresist layer.
Inorganic as well as organic fillers may also be added to the photoresist layer components to improve the mechanical properties of the cured film. Preferably, during preparation, the filler is the last component added just prior to thorough mixing, e.g. with a roll mill, to disperse the filler forming a homogeneous composition. Typical fillers used are surface treated oxides of silicon, aluminum and magnesium.
Thickness:
Thickness of the resin layer can range widely, in accordance with design requirements. U.S. Pat. No. 4,970,532, for example, discloses a thickness range of 20 to 200 microns. Variation from the desired thickness, however, must be minimized to maintain consistent ink drop volumes from each nozzle. U.S. Pat. No. 4,994,826 discloses a maximum tolerance of +/xe2x88x925% of resin layer thickness. Dry film photoresist materials offer consistent thickness in high volume over liquid resists and eliminate the costs of constant wafer-to-wafer quality assurance programs associated with spin-cast liquid resists.
The negative resin layers or resist materials are capable of aspect ratios of about 1.5 to 5 (i.e., 10 micron channels can be produced from films as thick as about 50 microns). Narrower channels can be produced with proportionately thinner films. Such resolution is dependent on several factors, such as the selected light source, photochemistry of the photoresist material, adhesion of the cured photopolymer to the first substrate, and selection of the development process, which includes the proper choice of developing liquid.
The resist layer is normally imaged with actinic radiation through a target which is registered with the underlying microresistors. Collimated light generally is used to obtain channel walls perpendicular to the bottom plate. Channel walls can be made either thinner or thicker at the top than at bottom, depending on the choice and balance of active ingredients.
The development process makes use of a difference in solubility of exposed and unexposed material. This difference in solubility can be maximized by the choice of developing liquid. The unexposed resist should show only moderate solubility in the developer. Solvents used for coating or stripping are generally too aggressive when used as developing solvents. When developing liquids are too aggressive, the photocured resist or resin layer tends to swell and peel from the substrate and development latitude is diminished.
Excessive stresses imposed during the development process can remove finer features of imaged resist or resin layer from the substrate and cause an apparent loss of resolution.
Photoimageable polyamic acid films can also be developed in an aqueous medium such as an aqueous solution of tetramethyl ammonium hydroxide, or aqueous solutions of triethanolamine or triethylamine.
The top plate is then bonded, preferably laminated, to the developed and thermally cured photoresist layer. The top plate may have a support plate and a surface layer of a noble metal. Some examples of support plates include glass, ceramics, metal, plastics, thermoplastic resins such as acrylic resins, ABS resins, polyethylene and the like. Some examples of noble metals include gold, platinum, palladium and iridium.
The various print head manufacturing processes differ in how each of these general steps is done. The photopolymerizable poly(amic acid) is spin coated on a bottom plate such as a silicon wafer. The wafer is soft baked at 80 to 105xc2x0 C. until the surface is tack dry. The dried wafer is exposed to actinic radiation through a positive or negative transparency and then developed and thermally cured at 150 to 400xc2x0 C., preferably 180 to 250xc2x0 C. In one embodiment, a noble metal-surfaced top plate may be aligned appropriately with features on the face of a bottom plate that bears printed microcircuits and an imaged and developed resin layer, and then bonded with the resin layer or resist surface using heat and pressure. After cooling, and cutting one or more ink jet print heads, with improved adhesion of the resist to the top and bottom plates, are produced.